Clothes sizes are a nightmare, they vary from one shop to another. A size 4 from Primark with be completely different to a size 4 for from M&S and bra sizes are even worse.
....and don't get me started on doll measurements from manufacturers/vendors
Seriously, the only way to get clothes that fit is by trial and error - stretchy stuff is your friend
That's one of the reasons I try and post the size and make of the outfits my girls wear in their shoots.
Psion